In general, when soldering each electronic part to an electrode (soldering portion) on a printed circuit board, solder paste is applied to the electrode on the printed circuit board by a printing method or a discharge method, and then the electronic part is soldered on the printed circuit board by pre-heating and main heating the printed circuit board in a chamber of reflow device.
The solder paste is made of a bonding material which solder powder is mixed with a flux. In recent years, residue-free solder paste where a cleaning step of flux residue in the soldering portion is unnecessary after reflowing is widely available in the semiconductor field, the automotive industry and the like. For example, Patent Document 1 discloses residue-free solder paste comprising a solid solvent which is solid at room temperature and which vaporizes at a reflow soldering temperature, a highly viscous solvent which is a highly viscous fluid at room temperature and which vaporizes at the reflow soldering temperature, and a liquid solvent which is liquid at room temperature and which vaporizes at the reflow soldering temperature as a balance.
In addition, as a measure against flux residue, the technology where an electronic part is soldered on the printed circuit board by using a fluxless preform material has been known other than the use of residue-free solder paste mentioned above. For example, Patent Document 2 discloses a power module where a fluxless solder preform material is used in bonding a semiconductor element to an insulating metal board. In this case, the reflow is conducted in a reducing atmosphere due to the use of the fluxless preform material.